1. Field of the Invention
The present invention relates to a speed change control system having a constant speed mode for causing a vehicle to travel at a constant speed, which vehicle includes a stepless speed change device for steplessly speed-changing a rotational power from an engine and outputting the speed-changed power.
2. Description of the Related Art
The constant speed mode for causing a vehicle to travel at a constant speed is called also a cruising mode. This mode is used when a work vehicle such as a tractor carries out a utility work such as a cultivating work, a ground leveling work, etc., while the vehicle travels at a constant speed. With using this constant speed mode, the driver or operator is free from the trouble of vehicle speed adjustment. A work vehicle having such constant speed mode is disclosed in Japanese unexamined patent application publication No. 2008-037400 (JP 2008-037400 A) (paragraphs [0027] through [00087], FIG. 6). This work vehicle mounts a hydrostatic stepless speed change device (“HST” hereinafter) as the stepless speed change device and realizes the constant speed travel by maintaining the swash plate angle of a hydraulic pump to a predetermined angle. For this reason, as operational tools for setting a swash plate target operational position as a speed change value for speed change controlling means for operating the swash plate angle of the hydraulic pump, there are provided a constant speed lever having a speed change lever with a friction type position maintaining tool. Of the swash plate target operational position set by the constant speed lever and the swash plate' target operational position set by the speed change lever, one which provides a higher vehicle speed corresponding thereto is set as the actual speed change value for the HST. Therefore, if e.g. the swash plate target operational position set by the constant speed lever is set to the zero speed in advance, the HST will be speed-changed according to an operational amount of the speed change lever, whereby the desired vehicle speed can be obtained. If the constant speed lever is maintained at an operational position for obtaining a predetermined constant travel speed, when the swash plate target operational position set by the speed change lever is lower than the swash plate target operational position set by the constant speed lever (this condition can be easily realized by the operator's keeping his/her foot away from the constant speed pedal thus keeping the pedal free), constant speed traveling of the vehicle at that desired constant travel speed is made possible.
However, with the constant speed mode control using a constant speed lever which is retained by means of friction, the friction makes accurate setting of a constant vehicle travel speed difficult. Especially, after release of the constant speed mode by lowering the constant speed lever to the zero speed position, it is difficult to set the constant speed mode to the same speed as before.
In view of the above-described state of the art, the object of the present invention is to provide a speed change control system for a vehicle capable of realizing a constant speed travel at a desired speed with a simple operation, with a simple arrangement.